The present invention relates to a magnetic device for correcting geometrical defects in the image created on the screen of a cathode-ray tube and is more particularly suited to tubes whose front face has a high radius of curvature.
A cathode-ray tube designed to generate colour images generally comprises an electron gun emitting three electron beams, each beam being designed to excite a luminescent material of a particular primary colour (red, green or blue) on the screen of the tube.
The electron beams scan the tube's screen under the influence of the deflection fields created by a deflection device, also called deflection yoke, fastened to the neck of the tube, comprising horizontal and vertical coils for deflecting the said beams. A substantially frustoconical-shaped ring, made of a ferromagnetic material, conventionally surrounds the deflection coils so as to concentrate the deflection fields in the appropriate region.
The three beams generated by the electron gun must always converge on the tube's screen or else suffer in the introduction of an error called a convergence error which, in particular, distorts the rendition of the colours. In order to achieve convergence of the three coplanar beams, it is known to use fields called self-converging astigmatic deflection fields; in a self-converging deflection coil, the lines of flux caused by the horizontal deflection winding are generally in the form of a pincushion in a portion of the coil which lies more to the front of the latter on the side of the screen of the tube. This amounts to introducing, into the distribution of the turns making up the line coil, a highly positive 3rd harmonic of the ampere-turns density at the front of the coil.
Moreover, due to the action of uniform horizontal and vertical magnetic deflection fields, the volume scanned by the electron beams is a pyramid, the apex of which is coincident with the centre of deflection of the deflection yoke and the intersection of which with a non-spherical screen surface exhibits a geometrical defect called pincushion distortion. This geometrical distortion of the image is all the greater the larger the radius of curvature of the screen of the tube. Self-converging deflection yokes generate astigmatic deflection fields making it possible to modify the north/south and east west geometry of the image and, in particular compensate for the north/south pincushion distortion. The east/west geometrical defects are generally corrected by an electronic circuit associated with the deflection yoke.
However, the current trend which is developing towards tubes having an increasingly flat, or even a completely flat screen surface particularly amplifies the image geometry problems; the result of this is that the self-convergent deflection yokes can no longer completely provide the geometrical correction for the north/south pincushion, while moreover, the east/west geometrical defects require increasingly strong corrections.
To correct these pincushion-shaped distortions of the image, linked to the flatness of the screen and to the self-convergent deflection device equipping the tube, it is known to use magnetic correction means in the form either of permanent magnets or of magnetic coils powered by a constant or variable current.
These magnetic correction means are generally borne by the front ring of the separator, and therefore located above the front bundle of the deflection coils. However, these solution have to generate increasingly strong correction fields and then lead to residual distortions such as image symmetry defects or else register defects which affect the purity of the colours on the screen.